This invention relates to a self-supporting unitary assemblage of criss-crossing rigid, porous, inorganic hollow filaments which can be used in such pressure-driven separation applications as ultrafiltration, hyperfiltration, and reverse osmosis.
Wound structures of criss-crossing flexible organic polymer hollow filaments are disclosed in U.S. Pat. Nos. 3,422,008 and 3,794,468 for such separation applications as ion exchange, reverse osmosis, wherein dissolved material is separated from the liquid (solvent) feed, and gas diffusion. The flexibility of these filaments which enables them to be wound up in criss-crossing fashion also limits the usefulness of the resultant structure in the separation application. For instance, when the separation is pressure driven; i.e., depends upon a pressure differential between the filament interior and exterior, the filament tends to collapse. Collapse resistance is obtained by making the filaments very fine, e.g., 0.005 inch (0.127 mm) and less, but this leads to plugging of the interior passage within the filament and moreover the filament is still subject to collapse when the pressure differential is increased to boost the flux (flow rate of liquid permeate or filtrate through the wall of the filament per unit of filtration surface area) of the separation process. Heat, chemical, and erosion resistance are also limitations on the use of the structures of organic polymer hollow filaments.
Rigid, porous, inorganic hollow tubes, such as of porcelain as disclosed in U.S. Pat. No. 3,664,507 or of carbon and alumina as disclosed in German OLS No. 2,422,477 which are capable of withstanding high pressure differentials and which overcome the heat, chemical, and erosion limitations of the organic hollow filaments have been developed for pressure-driven separation applications including those in which solid (undissolved) material is separated from the liquid feed. In both these references, the tubes are straight tubes, supported at one end in U.S. Pat. No. 3,664,507 and at both ends in German OLS No. 2,422,477. Such tubes are rigid, a property which enables them to withstand high pressure differentials without collapsing, but which has also limited the use of these tubes as straight lengths only. These straight lengths of tube limit the closeness with which the tubes can be packed together without blocking liquid from flowing to or from the thickness of the tube through which the separation operation occurs. In addition, the straight lengths of tube are brittle and lack toughness, which has to be compensated for by making the tubes of rather heavy construction, i.e., large diameter and wall thickness. For example, tube of 0.25 inch (6.35 mm) I.D. and a wall thickness of 0.06 inch is preferred in OLS No. 2,422,477 and 0.21 inch (5.33 mm) O.D. tube is disclosed in U.S. Pat. No. 3,664,507. The limitation on tube packing and requirement of heavy tube construction provides a relatively low tube surface area per unit of volume, which means that rather large equipment is needed to house the tubes, depending on the surface area required for a particular application. The heavy construction of the tube also has to compensate for the weakening effect of the pores that must be present in the tube wall for the permeation of liquid therethrough, and the porosity of the tube must be limited accordingly.
The relatively heavy construction of the straight tubes is generally a compromise between the sacrifice in surface area per unit of volume and porosity and the strength needed for handling and use, which still requires great care in the handling and use of the tubes in order to avoid breakage. In this regard, the tubes are generally supplied separate from one another in the sense that they are individually inserted into the separation equipment. This separate handling of straight tubes is a delicate operation which can lead to tube breakage and added cost.
The relatively heavy construction of the tubes also has to compensate for the mounting arrangement of the tubes within the separation equipment. In this regard, the tubes are mounted to extend in cantilever fashion from a tube sheet and may be supported at their opposite ends. In any event, the tubes are subject to flexure and breakage by the liquid flowing through the equipment. This limits the lengths of the tubes that can be used. It is also usual for the liquid flow when supplied from the exterior of the tubes to be along the axis of the tubes, rather than transverse thereto, to minimize tube flexure and breakage. Liquid flow supplied to the interior of the tubes is also along the axis of the tubes to minimize tube flexure.
In the ultrafiltration application, the straight rigid, porous, inorganic hollow tubes have shown another disadvantage. In this separation application, a measure of efficiency is the consumption of power used in pumping liquid along the filter surface (rather than pressure differential between the interior and exterior of the tube) vs. the amount of filtrate collected of desired quality for this power consumption. For a given set of process conditions, the operating flux of the straight tubes becomes very low, with the result that the power consumed for the amount of filtrate obtained is higher than desired. By "operating flux" is meant the flux of the tubes after the initial sharp decrease in flux occurs at the beginning of the ultrafiltration and the separation system is considered to be operating. The operating flux declines more gradually than this initial sharp decrease in flux. Although operating flux decline is gradual, such flux eventually reaches the point where it becomes economical to even stop the filtration operation in order to backwash the tubes, which improves the operating flux but only temporarily. The ability to retain the initial flux of the operating flux is called flux retention. The straight tubes generally have a low flux retention. Flux retention may be high, however, if the operating flux is so low to begin with that there is little room for decrease in flux with the passage of time.
The low operating flux and/or poor flux retention of the straight tubes is due to the buildup of particulate matter from the liquid being filtered on the tube surface and buildup of a stagnant fluid boundary layer on the tube surface. The generally axial flow of the liquid used in connection with these tubes so as not to break them is a relatively laminar flow of liquid. This laminar flow is not very effective in washing away the buildup of particulate matter or stagnant fluid boundary layer. The same is true for the buildup of the boundary layer of liquid concentrate when the straight tubes are used for hyperfiltration or reverse osmosis.
D. G. Thomas et al., "Turbulence Promoters for Hyperfiltration with Dynamic Membranes", Environmental Science and Technology 4, 1129-1136, Dec. 1970, disclose the use of devices preferably mounted within the interior of straight porous carbon tubes for promoting turbulence in the flow of liquid fed to the hollow interior of the 0.39 inch (9.9 mm) O.D. tubes (wall thickness of 2 mm), which had the effect of increasing the flux of the tubes (including the dymanic membrane formed thereon) by as much as 150% for the particular system tested. Unfortunately, the economics of operating these tubes with turbulence promoters is less than desirable because of the need to make the tubes large enough to fit the turbulence promoters inside them, which detracts from surface area per unit of volume, and the turbulence promoters themselves act as sites for buildup of particulate matter, which detracts from the surface of the tubes available for filtration and eventually may lead to the shutting down of the filtration operation for replacement of the turbulence promoters.
R. E. Lacey and S. Loeb, Industrial Processing with Membranes, published by Wiley-Interscience (1972) disclose on pages 257 to 259 a belief that velocity of the fluid (liquid) can minimize the deposition of a boundary layer of ionic and collodial materials that decrease flux and advance a concept of a staggered membrane configuration which provides impinging flow and which breaks up the flow before buildup of a stable boundary layer. The authors note that the membrane support configurations which provide flow break up were not observed in the literature (page 258) and do not make any proposals for such configurations themselves. The authors also note the disadvantage of inserted turbulence promoters and say "A new and fresh approach is needed" (page 259).